GSK3B and Thrombocytopenia: This side effect was considered a potential sign of target engagement and a drug class effect, due to the known prevalence of GSK-3B in the synaptic layers of the retina and photoreceptor cells.40 Side effects from lomustine included grades 3/4 thrombocytopenia (3/14, 21%) and grade 1/2 fatigue (4/14, 28%) as expected.